The aim of this study is to clarify the mechanism of absorption of water-soluble vitamins in mammalian small intestine. The prevailing textbook concept of vitamin absorption by simple diffusion may be inaccurate. The unidirectional influx of individual vitamins across the brush border will be investigated in vitro on appropriate segments of small intestine from guinea pig, hamster, rat and rabbit; comparative studies on human intestine will be performed following identification of the optimal experimental conditions in the other species. We will look for evidence of a carrier-mediated transport process at the brush border by measuring vitamin influx from the mucosal solution into the cell interior over a wide range of initial substrate concentrations in the mucosal solution. The possibility of Na-dependent transport will be evaluated by measuring vitamin influx with various concentrations of Na in the extracellular mucosal solution and by artificially lowering the intracellular Na concentration prior to measuring influx. Vitamin influx will be measured with other actively transported substrates (sugars and amino acids) in the mucosal solution to evaluate the possibility of competition between substrates which might indicate a common transport mechanism. Vitamin influx will also be measured following application of metabolic inhibitors to the tissue to determine if cellular metabolic energy is directly required to mediate the influx process. And finally, in cases where an active transport process appears likely, studies to demonstrate cellular accumulation against an electrochemical gradient and net transport from mucosa to serosa will be performed to further characterize the transport mechanism. The occurrence of specific transport mechanisms for vitamins in intestine would allow for the possibilities of regulation of transport rate and competition between vitamins for transport sites, concepts of possible concern for individuals who take supplemental vitamins. Evidence for feed-back regulation of uptake rate and possible competition between vitamins for transport will be sought in experimental animals.